Chemokines, also referred to as intecrines, are soluble, low molecular weight members of the cytokine family which have chemoattractant function. Chemokines are capable of selectively inducing chemotaxis of the formed elements of the blood (other than red blood cells), including leukocytes such as monocytes, macrophages, eosinophils, basophils, mast cells, and lymphocytes, such as T cells, B cells, and polymorphonuclear leukocytes (neutrophils)). In addition to stimulating chemotaxis, other changes can be selectively induced by chemokines in responsive cells, including changes in cell shape, transient rises in the concentration of intracellular free calcium ([Ca2+]i), granule exocytosis, integrin upregulation, formation of bioactive lipids (e.g., leukotrienes) and respiratory burst, associated with leukocyte activation. Thus, the chemokines are early triggers of the inflammatory response, causing inflammatory mediator release, chemotaxis and extravasation to sites of infection or inflammation.
The chemokines characterized to date are related in primary structure. They share four conserved cysteines, which form disulphide bonds. cDNA cloning and biochemical characterization of several chemokines has revealed that the proteins have a leader sequence of 20–25 amino acids, which is cleaved upon secretion to yield a mature protein of approximately 92–99 amino acids. Based on the conserved cysteine motif, the family is divided into two branches, designated as the C—C chemokines (β chemokines) and the C—x-C chemokines (α chemokines), in which the first two conserved cysteines are adjacent or are separated by an intervening residue, respectively. Baggiolini, M. and C. A. Dahinden, Immunology Today, 15: 127–133 (1994)).
The C—X—C chemokines include a number of chemoattractants which are potent chemoattractants and activators of neutrophils, such as interleukin 8 (IL-8), PF4 and neutrophil-activating peptide 2 (NAP-2). The C—C chemokines include molecules such as human monocyte chemotactic proteins 1–3 (MCP-1, MCP-2 and MCP-3), RANTES (Regulated on Activation, Normal T Expressed and Secreted), and the macrophage inflammatory proteins 1α and 1β (MIP-1α and MIP-1β), which have been characterized as chemoattractants and activators of monocytes or lymphocytes, but do not appear to be chemoattractants for neutrophils. For example, recombinant RANTES is a chemoattractant for monocytes, as well as for memory T cells in vitro (Schall, T. J. et al., Nature, 347: 669–671 (1990)). More recently a chemokine called lymphotactin with a single cysteine pair in the molecule has been identified which attracts lymphocytes (Kelner, G. S., et al., Science, 266: 1395–1359 (1994)).
The C—C chemokines are of great interest because of their potential role in allergic inflammation. For example, MCP-1 induces exocytosis of human basophils, resulting in release of high levels of inflammatory mediators, such as histamine and leukotriene c4. Similarly, there is great interest in the receptors for the C—C chemokines, which trigger these cellular events in response to chemokine binding. A receptor for C—C chemokines has recently been cloned and is reported to bind MIP-1α and RANTES. Accordingly, this MIP-1α/RANTES receptor was designated C—C chemokine receptor 1 (CKR-1; Neote, K. et al., Cell, 72: 415–425 (1993); Horuk, R. et al., WO 94/11504, published May 26, 1994; Gao, J. -I. et al., J. Exp. Med., 177: 1421–1427 (1993)). An MCP-1 receptor has also been cloned (Charo, I. F. et al., Proc. Natl. Acad. Sci. USA, 91: 2752 (1994)). This receptor, designated CKR-2, is reported to bind MCP-1 with high affinity and MCP-3 with lower affinity (Charo, I. F., et al., Proc. Natl. Acad. Sci. USA, 91: 2752–2756 (1994)). CKR-2 has been shown to exist in two isoforms resulting from the use of an alternative splice site in isoform A producing a distinct cytoplasmic tail. Isoform B, which is not spliced in this region, has been shown to be a functional receptor for MCP-1 and MCP-3 in binding and signal transduction assays (Charo, I. F., et al., Proc. Natl. Acad. Sci. USA, 91: 2752–2756 (1994); Myers, S. J., et al., J. Biol. Chem., 270: 5786–5792 (1995)). More recently, a new receptor called CKR-4 has been described; cRNA from this receptor was reported to produce a Ca2+ activated chloride current in response to MCP-1, MIP-1α, and RANTES when injected in to X. laevis oocytes (Power, C. A., et al., J. Biol. Chem., 270: 19495–19500 (1995)).
The MCP-1 receptor (CKR-2) and C—C chemokine receptor 1 are predicted to belong to a superfamily of seven transmembrane spanning G-protein coupled receptors (Gerard C., and Gerard, N. P., Annu. Rev. Immunol., 12: 775–808 (1994); Gerard C., and Gerard N. P., Curr. Opin. Immunol., 6: 140–145 (1994)). This family of G-protein coupled (serpentine) receptors comprises a large group of integral membrane proteins, containing seven transmembrane-spanning regions. The ligands of these receptors include a diverse group of molecules, including small biogenic amine molecules, such as epinephrine and norepinephrine, peptides, such as substance P and neurokinins, and larger proteins, such as chemokines. The receptors are coupled to G proteins, which are heterotrimeric regulatory proteins capable of binding GTP and mediating signal transduction from coupled receptors, for example, by the production of intracellular mediators.
The cloning and sequencing of two IL-8 receptor cDNAs reveals that these C—X—C receptor proteins also share sequence similarity with seven transmembrane-spanning G protein-coupled receptor proteins (Murphy P. M. and H. L. Tiffany, Science, 253: 1280–1283 (1991); Murphy et al., WO 93/06299; Holmes, W. E. et al., Science, 253: 1278–1280 (1991)). Additional receptors for chemotactic proteins such as anaphylatoxin C5a and bacterial formylated tripeptide fMLP have been characterized by cloning and been found to encode receptor proteins which also share sequence similarity to these seven transmembrane-spanning proteins (Gerard, N. P. and C. Gerard, Nature, 349: 614–617 (1991); Boulay, F. et al., Biochemistry, 29: 11123–11133 (1990)). Although a number of other proteins with significant sequence similarity and similar tissue and leukocyte subpopulation distribution to known chemokine receptors have been identified and cloned, the ligands for these receptors remain undefined. Thus, these proteins are referred to as orphan receptors.
The isolation and characterization of additional genes and the encoded receptors, and the characterization of the corresponding ligands, is essential to an understanding of the interaction of chemokines with their target cells and the events stimulated by this interaction, including chemotaxis and cellular activation of leukocytes.